In the vast firmament of science and technology, terahertz technology shines like a rising star. Endowed with its unique physical properties, it demonstrates boundless application potential in numerous cutting-edge fields. Terahertz waves have a frequency range between 0.1 THz and 10 THz; signals in this frequency band possess high penetrability, allowing them to easily pass through common materials such as paper, plastic, wood, and some ceramics. Meanwhile, they exhibit non-ionizing characteristics and do not cause radiation damage to biological tissues. These two key properties endow terahertz technology with unique appeal in fields like biomedical imaging, biosensing, and non-destructive testing, attracting a host of researchers and enterprises to engage in related studies and applications. 01 Dilemmas in Terahertz Imaging and the Need for Breakthroughs In the biomedical field, the accurate identification and observation of early-stage cancerous tissues have long been a key focus and challenge in medical research. Terahertz waves can detect subtle changes in water content and differences in molecular structures within biological tissues, offering a brand-new technical approach for the detection of early-stage cancer. In terms of biosensing, terahertz waves match the vibrational and rotational energy levels of specific molecules, enabling precise identification of particular chemical components. In the field of non-destructive testing, terahertz technology can effectively detect internal defects in composite materials, such as microcracks and air bubbles, thus ensuring the quality and safety of materials. However, the development of terahertz imaging systems has not been smooth sailing. Traditional dielectric lenses are like a heavy boulder, seriously hindering the advancement of terahertz imaging technology. The severe chromatic aberration they suffer from means that terahertz waves of different frequencies cannot converge at the same point after being refracted by the lens, resulting in blurred imaging. Additionally, strong spherical aberration causes light at the edge of the lens to fail to focus accurately, further degrading imaging quality. Moreover, low resolution limits the ability of terahertz imaging systems to distinguish fine structures and features. These issues create a huge gap between the quality of terahertz imaging and the requirements of practical applications. Especially for imaging in the high-frequency band above 0.3 THz, developing solutions for ultra-high-resolution imaging systems has become an urgent priority. 02 The Birth of an Innovative Metalens At this critical juncture, the research team from the State Key Laboratory of Terahertz and Millimeter Waves at City University of Hong Kong took on the challenge. They successfully developed an ultra-broadband, achromatic, ultra-resolution, and wide-angle terahertz imaging lens, and conducted highly valuable application demonstrations in biological imaging and non-destructive testing. The relevant research results were published in the international top academic journal *Nature Communications* under the title "3D-printed aberration-free terahertz metalenses for high numerical aperture, ultra-broadband achromatic super-resolution wide-angle imaging," attracting widespread attention in the scientific community. The core of this research lies in the innovative proposal and design of radially gradient periodic metamaterials. By carefully designing and regulating the microstructures of the material, these metamaterials impart special phase and amplitude changes to terahertz waves during their propagation. This enables ultra-resolution imaging within an ultra-high operating bandwidth while cleverly eliminating chromatic aberration and coma—two key factors that hinder imaging quality. The structure of this terahertz metalens is complex yet precise, composed of radially gradient periodic metamaterials. To accurately realize its complex structure, the research team utilized the high-precision microArch® S230 3D printing system from MicroFabrica. With an ultra-high precision of 2 μm, this system is like a highly skilled craftsman, accurately transforming the design blueprint of the metalens into a physical product, laying a solid material foundation for the realization of the metalens' performance. Building on the terahertz metalens, the research team further applied innovative thinking by placing two terahertz metalenses symmetrically along the focal point, constructing a new terahertz ultra-resolution imaging system. This innovative design is like a stroke of genius—through an ingenious optical layout, it improves the resolution by 1.5 times based on the focusing accuracy of a single terahertz metalens. This significant improvement endows the system with a stronger ability to distinguish submillimeter-level tissue details, providing strong support for subsequent applications in fields such as biological imaging and non-destructive testing. 03 Comprehensive Verification of the Metalens' Performance To conduct a comprehensive and in-depth verification of the terahertz metalens' performance, the research team carried out a series of rigorous and detailed experiments. When measuring the electric field distribution in the range of 0.2 THz to 0.9 THz, they used an Anritsu MG3697C signal generator to produce radio frequency input signals. These signals underwent precise processing through a series of Signal Generator Extender (SGX) modules and multipliers, generating linearly polarized THz signals covering the range of 0.2–0.9 THz. At the receiving end, a Signal Analyzer Extender (SAX) module was used, which was fixed on a two-dimensional motorized stage. Driven by a computer-controlled stepper motor, this stage moved with a fine step size of 0.1 mm. The received THz signals were processed and recorded using a KEYSIGHT N9030A signal analyzer, thereby accurately obtaining two-dimensional electric field distribution data. For the measurement of off-axis focusing, the research team carefully designed a turntable device. The SGX module and multiplier were installed on the turntable and precisely positioned at the center of the metalens. This clever arrangement allowed flexible adjustment of the incident angle of THz waves, enabling full-range measurement from 0° to 90° and providing abundant data for studying the focusing performance of the metalens at different angles. The imaging experiments were equally remarkable. The operating principle of the imaging system is as follows: a diagonal horn antenna emits y-polarized THz waves, which are incident on Metalens 1 (M1) and form a focal point on the sample. Subsequently, Metalens 2 (M2) collimates the focused beam generated by M1, and finally, the signal is collected by a receiving (Rx) horn antenna. Compared with a single-metalens configuration, this dual-metalens setup greatly improves imaging resolution. In the experiment, the operating frequency was set to 0.7 THz—a choice determined as the optimal balance after repeated trade-offs and experimental verification between the full width at half maximum (FWHM) and efficiency. The imaging sample was placed on the confocal plane of M1 and M2 and mounted on a computer-controlled motorized stage, which performed 2D raster scanning through precise movement. The collected 2D power data underwent complex post-processing, ultimately generating a clear mapped image of the scene. Before evaluating the imaging performance, the research team also conducted a strict calibration procedure. This procedure involved evaluating total losses—including path losses and conversion losses introduced by VDI equipment—without the metalens and sample installed. The measurement data of the sample was then deconvolved to effectively offset inaccuracies during the measurement process and significantly enhance imaging contrast. To fully demonstrate the wide-angle imaging performance, the system adopted a 360° rotation stage in the xz-plane. The symmetrically installed transmitter (Tx) and receiver (Rx) devices allowed the system to achieve incident angles (θ) of any value, ensuring a high degree of flexibility for off-axis imaging. The metalens was precisely positioned at the center of the rotation stage, and at the same time, the displacement between the two lenses (dx and dz) was carefully adjusted based on the focal length offset evaluated in previous experiments, ensuring optimal focusing at any incident angle. In the imaging experiments, the research team selected precision circuit structures hidden under dielectric plates, grating structures with defects, and fresh leaves as imaging samples. The experimental results were impressive: the system could accurately identify tiny defects of the order of 0.1 mm in grating structures, clearly distinguish microstrip lines with a spacing of only 0.2 mm in circuits, and also present the textile-like stripes inside dielectric plates and the fine tissue textures in leaves. In summary, the ultra-broadband, achromatic, ultra-resolution, and wide-angle terahertz imaging lens developed in this study represents a major leap forward in the field of terahertz imaging technology. For the first time, it achieves achromatic ultra-resolution focusing with a numerical aperture of 0.555 within the ultra-broadband range of 0.2 to 0.9 THz, featuring a large field of view of 90 degrees and high-resolution identification of targets with a spacing of 0.2 mm. This achievement successfully overcomes a series of long-standing technical challenges in the academic community, such as the complexity of achromatic lens systems, low resolution, incompatibility between large numerical apertures and large operating bandwidths, and incompatibility between achromatism and coma correction. It opens up a brand-new technical path for the development of next-generation compact and integrable terahertz imaging systems. In the future, this achievement is expected to promote the wide application of terahertz imaging technology in fields such as biomedicine and non-destructive testing, injecting strong impetus into the development of related fields and bringing more surprises and breakthroughs.